The present invention relates to novel, modified castor oil lubricating compositions for use in centrifugal compressor refrigerant systems and, in particular, to lubricating compositions having high lubricity which are thermally and chemically stable in the presence of and in contact with partially or completely fluorinated halocarbon gas refrigerants, said refrigerants being only slightly soluble in the lubricating compositions. The term halocarbon is herein used to mean hydrocarbon compounds having fluorine and chlorine atoms substituted for a high proportion or all of the monovalent hydrogen atoms on the carbons.
Refrigerant systems utilizing halocarbon refrigerants, such as dichlorodifluoromethane require specialized lubricants. These lubricants must be resistant to thermal and chemical decomposition at high temperatures present during gas compression, in the presence of the halocarbons.
In providing air conditioning for office buildings, stores, apartments and motels, for example, it is desirable and important to provide quiet, low vibration compressors that are compact and occupy the smallest possible space for the power needed to provide the requisite heat removal under expected conditions. Many of these air conditioning units employ chilled water, produced by the heat exchanger associated with the compressor, to effect suitable conditioning of the air in the building.
Piston type units are not only relatively large for a given horsepower, but they are noisy and vibrate. Centrifugal compressors driven by, for example, 50 to 600 horsepower electric motors have been found to be much more compact, so that they occupy only a fraction of the space required for a piston type unit of the same horsepower. Furthermore, considering the horsepower, the high speeds of up to 36,000 rpm of the centrifugal compressor, and large volumes of refrigerant handled per unit time, the compressor units are extremely quiet and are characterized by very little vibration.
However, a serious problem has been encountered in the starting of centrifugal compressors. The start-up of a centrifugal unit from a cold condition, normally 15.degree. C. to 24.degree. C., to a fully operational condition has often taken several hours. Under all conditions, a separate oil pump unit is first set in operation to deliver a flow of lubricating oil to the bearings, gears and oil-operated control mechanism; and only after an adequate flow of lubricant has been established, is the centrifugal compressor put into operation. Initial high thrust loads are encountered in the impeller bearings requiring good lubricant films to be present at all times when in operation.
This prolonged delay in a cold start occurs because of the high solubility of the halocarbon refrigerant, usually refrigerant 12, dichlorodifluoromethane, hereinafter referred to as R-12, in any of the otherwise satisfactory lubricating petroleum base oils used for lubricating the bearings and gearing of the centrifugal compressor. The halocarbon refrigerant comes into contact with the lubricant in the normal operation of the centrifugal compressor.
Large volumes of halocarbon gas dissolve in cold oil because the solubility of the halocarbon gas increases as temperature drops, and when the oil is being pumped to the compressor rotor and bearings, the dissolved halocarbon refrigerant readily boils out as a gas as a result of even small changes in pressure or temperature. Frequently, the oil or lubricant is flushed from the bearings during shutdown so that the bearing is dry and presents a highly undesirable dry metal to dry metal contact condition at the time start-up is required.
On a cold start-up, oil in the oil sump is saturated with halocarbon, which drastically dilutes the oil, and which halocarbon boils out of the oil lubricant to produce large volumes of foam both in the sump and in the oil lines, as well as in the bearings and at other places in the oil circuit when the oil pump is set into operation to convey oil or lubricant to the bearings, gears, and elsewhere. Unless the oil is still hot from previous use, insufficient oil will flow to the bearings, and at most, an initial halocarbon-oil foam is present which is inadequate to accomplish effective lubrication.
Failure of the bearings will occur if the compressor motor is started under these poor lubricating conditions. Further, the viscosity of the oil is reduced seriously by the dissolved halocarbon, so that the lubrication properties of the oil are deleteriously modified by this unwanted dilution. This is in addition to the danger that a sudden release of gas in the oil film on the bearing surfaces will cause a partial oil film failure which permits bare metal to bare metal contact with the potential for bearing damage.
At the present time, one involved procedure to mitigate this lubrication problem, in centrifugal compressors, is to provide a heater in or about the oil sump--so that the oil will be heated up to and maintained at, for instance, 65.degree. C. to minimize the amount of the halocarbon refrigerant, such as R-12, in solution in the oil.
In order to avoid the continual use of the heaters for lengthy shutdown periods, at start-up the oil sump is initially heated for several hours (using for instance 5 KW heaters) in order to drive out as much halocarbon from the progressively heated oil as is reasonably possible before actual operation of the oil pump of the compressor. The oil pump is then energized to pump the hot oil with low halocarbon content through the oil lines and into the bearings.
The chemical stability of the lubricants for a centrifugal refrigeration compressor is an important factor, since the systems are hermetically sealed and any reactions with the halocarbon refrigerant which cause deterioration of the lubricant so that it decomposes, and fails to provide adequate lubrication or reacts to form solids which will plug up tubing and orifices, as well as lead to its failure to function effectively as a lubricant, is fatal to the compressor system. Metals such as iron, aluminum and copper used in compressors are commonly in contact with the lubricant, and the halocarbon, of course, dissolves in the lubricant. This combination of materials at elevated temperatures can react adversely to cause the oil to ultimately fail.
The overall lubrication and start-up procedure would be greatly simplified by the existence of a lubricant which had a low affinity for halocarbon refrigerants, such as R-12, i.e., a lubricant in which R-12 is relatively insoluble, or as a minimum, in which R-12 or other halocarbon is slowly dissolved. Such a lubricant, as pointed out above, would permit much more rapid and reliable cold start up, and would be an improvement over known materials if it would also retain chemical and thermal stability in the presence of R-12.
Williamitis, in U.S. Pat. No. 2,807,155, recognized problems of thermal stability of lubricant systems in contact with a chlorodifluoromethane refrigerant, in refrigeration apparatus. He used pentaerythritol esters, dipentaerythritol esters, and tripentaerythritol esters which were highly soluble in the refrigerant, and had viscosities of up to 2,000 SUS, as the sole chemical and thermally stable lubricant. Mills et al., in U.S. Pat. No. 3,715,302, achieved outstanding chemical and thermal lubricant stability, in an R-12 refrigerant environment, by using a blend of hydrorefined naphthenic oil and refined and dewaxed paraffinic oil. This blend had a viscosity of up to 500 SUS at 100.degree. F., and was miscible in fluorinated hydrocarbon refrigerants such as R-12. Luck and Gainer, in U.S. Pat. No. 3,878,112, solved refrigerant solubility problems by using glycol diricinoleates as synthetic lubricants for centrifugal refrigeration compressors. These materials have a low solubility for fluorocarbon refrigerants but they are expensive and difficult to make in a highly pure state.